CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) are often used to ensure that information submitted to a computer system was submitted by a human rather than a computer. A CAPTCHA is a type of challenge-response test used to ensure that a response is not generated by a computer and are commonly used to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. The authentication process usually involves the system requesting a user to complete a simple test which the system is able to generate and grade. Assuming attacking computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. The most common type of CAPTCHA in use requires the user enter letters or digits from a distorted image that appears on a display screen.
Automated approaches to defeat the use of CAPTCHAs usually involve the use of optical character recognition (OCR). OCR is used by the attacking computer to “read” the letters or digits by analyzing a captured image of the CAPTCHA. The use of OCR has been countered by distorting the letters and digits forming the CAPTCHA in such a way that OCR can not solve the CAPTCHA, but humans can still perceive the letters and digits. In the meantime, OCR techniques have advanced to the point where standard distorted text-based CAPTCHAs require so much deformation to authenticate the user that the CAPCHAs in many cases are difficult for humans to read.
It is in this context that embodiments of the invention arise.